STORY OF SLATE
By Charles H. Davis
President, Davis Slate & Manufacturing Company
Director t National Slate Association, Philadelphia
A LECTURE MR. DAVIS DELIVERED BEFORE THE COMMERCIAL GEOGRAPHY CLASSES OF THE UNIVERSITY OF CHICAGO, NORTHWESTERN
UNIVERSITY AND THE CHICAGO HIGH SCHOOLS. PUBLISHED IN PRINTED FORM IN RESPONSE TO NUMEROUS REQUESTS FOR COPIES
The Story of Slate
To understand slate properly one
must know something of its origin
and must be able to follow in some
measure the slow but powerful
geologic forces that have shaped it into its
The natural history of slate falls into two
great periods, each period covering thousands
and possibly millions of years.
The first period has to do with the origin
of the mineral particles of which slate is
All rocks slowly wear away and disinte-
grate through the action of frost and water.
Sometimes fiction writers, orators or poets
speak of the everlasting hills — but no hills
are everlasting — the rocks that form them
are slowly but continuously changing into
new forms that are carried away and are
built up into other rocks.
Many minerals, particularly the feldspars,
slowly change into clay and this clay is
washed down by the rain, carried by streams
into rivers, and by rivers to the sea.
Clay particles are very small and light, so
are carried far from shore and deposited in
uniform beds on the ocean floor.
Such clay beds are the raw materials from
which slate is produced by the forces of
The second great period comprises the cen-
turies during which the unseen forces of
nature convert the clay into slate as we now
find it in the earth.
The clay beds are first compressed into a
firm mass known as shale.
The shales may later be intensely folded
It is generally assumed that the earth is
slowly cooling, and as it cools, it shrinks.
This shrinkage causes a wrinkling or fold-
ing of the surface rocks just as the skin of an
apple wrinkles when it is baked.
When the shales are caught in these folds
they are squeezed with terrific force and
under high temperature.
The forces are so great that in some
mysterious manner the clay is converted into
new minerals, the chief of which are mica
and chlorite, both consisting of minute
flakes and scales.
Millions of these tiny flakes overlap each
other like shingles on a roof.
As they all lie in one plane there is de-
veloped a remarkable tendency for the rock
to split with great ease in one direction.
This splitting tendency or cleavage is the
most important property of slate.
Slabs 4x6 feet in size, or larger, may be
readily split to less than J^-inch thick, and
there have been instances where sheets 8 x 16
inches have been split to the remarkable
thinness of -^-inch.
The little tabular or flake-like grains are
cemented under intense pressure which gives
slate unusual strength.
While human hands can take slate apart
they can never put the grains together again
with the same regularity, nor can they give
it the same strength as is possessed by the
The fineness of grain, uniformity and
strength of slate fit it peculiarly well for many
Remember now that no rocks are everlast-
ing and slate is no exception to this rule.
It is noteworthy, however, that slate con-
sists of non-metallic minerals that are very
enduring and that resist remarkably well the
forces of the weather that are ever at work
in wearing down the rocks.
Now what are some of the uses for which
this remarkable product is best adapted?
Its enduring properties and its tendency
to split into thin sheets led to its early use for
making weatherproof roofing for houses, and
this is still one of its important uses.
Black, gray, green, red, purple or mottled
slates form many of the most beautiful roofs
to be found in the world.
It has also been found that high-grade
slate is an excellent nonconductor of electric-
ity, and as it can be readily shaped, drilled
and polished, it is widely used for electrical
Engineers in power-houses and factories
take great pride in the attractive appearance
of the large switchboards which form an
important part of the equipment wherever
electricity is generated or used.
Slate slabs are attractive, and as they are
easily matched in color a slate switchboard
may be enlarged without detracting from its
The roof is not the only part of a building in
which slate is widely used.
It may be seen in many buildings in the
form of stair steps, baseboards, window sills,
shower stalls, floor tile, etc.
It is also fashioned into laundry tubs,
water tanks, kitchen sinks, dough troughs in
bakeries and similar furnishings.
You are all familiar with slate in the form
of blackboards, a use for which it excels all
It is claimed that one area in Pennsylvania,
26 miles long and 2 or 3 miles wide, provides
most of the blackboard slate in the world.
Our grandparents used school slates, but
they have been largely replaced in the United
States by exercise books, more commonly,
and I hope incorrectly, called "scribbling
School slates are still manufactured in
great quantities, but about 90 percent of
them are exported to foreign lands.
You have now had a brief outline of the
origin and history of slate and its chief uses.
Let us turn to a natural ledge of slate rock
as it lies in the mountain side, and follow
each step in the process of shaping it into
Slate occurs in many parts of the country
and is worked chiefly in the states of
Pennsylvania, New York, Vermont, Maine,
Maryland and Virginia.
When a deposit of good slate is found the
chief task is to remove all the soil or inferior
rock from the surface leaving the good slate
The slate may be removed by blasting in
drill holes, but many operators now use
channeling machines to cut out massive
A channeler is a machine operated by
steam or compressed air that chops the rock
by repeated blows of heavy chisel-like bars.
In this way a vertical channel or groove 2
or 3 inches wide and possibly 12 feet deep is
cut along the wall of the quarry, and cross
channels subdivide the rock into large rec-
The latter are broken free from the quarry
floor by driving wedges in drill holes or by
the discharge of a small amount of black
The large masses are subdivided by split-
ting along the direction of slaty cleavage and
by making fractures in other directions with
wedges driven into drill holes.
Masses of slate thus obtained weighing 1
to 3 tons are hoisted to the surface.
Great chains are placed about them and
they are elevated with steel cables wound on
drums by powerful hoist engines.
Some slate quarries have been worked for
many years and thus are of wide extent and
Several of the Pennsylvania quarries are
more than 400 feet in depth.
The treatment that a block of slate receives
after it is removed from the quarry depends
entirely on the purpose for which it is to be
If it is to be converted into roofing slates
the process is very simple.
The block is first subdivided into masses
24 to 30 inches in length and the thickness of
This is the task of the block-maker.
The splitter then takes these masses and
with a thin flexible steel chisel and a wooden
mallet he subdivides each block, always
splitting it in the center, until the 8 slates
The trimmer then places them on a trim-
ming machine where the irregular edges are
cut away leaving the largest perfect rectangle
of a given standard size that the slab will
Trimming is done with a heavy blade like a
great meat cleaver, operated with a foot
treadle, or sometimes with a curved rotating
blade like that of a lawn mower.
The finished slates are piled in racks
according to size, and when holes are punched
in them for nailing they are ready to be placed
on a roof.
Slate used for other purposes is usually
termed "structural slate" or " milled stock."
Mills equipped with various machines are
required for the manufacture of structural
The first milling process is most surprising.
You have all at some time or other ob-
served a circular saw cutting logs into lumber
or boards into shorter pieces.
You may have wondered at the ease with
which a circular saw eats its way through a
hard piece of timber, but it seems even more
remarkable that a circular saw can be used
to cut a piece of rock.
Such is the case, however, for the first
operation in the mill is to place the block of
slate on a heavy traveling bed that carries
it against the teeth of an especially designed
The bed travels very slowly, and the saw
rotates at much slower speed than a wood
saw, but a mass of slate 1 foot thick and 4
feet wide may be cut across in a very few
The blocks obtained may be split to the
desired thickness for blackboards.
If structural slabs are desired the blocks
are placed on a second traveling bed and
passed repeatedly beneath a heavy blade
which scrapes the surface smooth. This ma-
chine is called a "planer."
The surfaces may be sand-rubbed and
polished by other machines.
Edges may be trimmed or bevels cut with
In a properly arranged slate mill the rough
block enters at one end and passes from one
machine to another in a regular order until
it is prepared for shipment.
An interesting feature of slate-working is the
importance of keeping the block moist until
final splitting is accomplished.
As the block lies in its natural bed it con-
tains what is called "quarry water."
If this water once dries out, the slate will not
split readily, and no amount of wetting will
renew the splitting property if once it is lost.
All quarry blocks are conveyed directly to
the mill, and if there is any long delay water
is thrown over them or they are covered to
Only the highest quality of slate is used
for the manufacture of slate products, and
all defective blocks are thrown away.
Thus great mountains of waste are built up
around slate quarries, and one of the big
problems in the industry is to find uses for
this waste material.
Some of it is ground to a fine dust and
mixed with asphalt for making roads. It is
also used in such products as floor linoleum,
rubber and paint.
From the brief historical outline presented
you will appreciate that slate is a remarkable
material which nature has endowed with
peculiar and valuable properties.
This short description of uses and proc-
esses shows you that man has invented
methods whereby these unusual properties
may be turned to practical use with the result
that slate furnishes many useful products
which contribute to comfort and convenience.
Sometimes on foggy mornings objects appear
with dim outlines, and with difficulty we
distinguish a house from a tree.
We see, but our vision is not clear.
On the other hand we sometimes see ob-
jects plainly in so far as our actual eyesight
is concerned, but the object conveys little
impression to our minds because it is not
understood — our lack of information about
the object covers it like a fog, and we see
only its outlines.
Possibly with most of you slate is one of
those objects which you see with the eye
only, and this short story will have served its
purpose if it clears away the fog in some
measure and enables you, when you see slate
with the eye, to see it also with the under-
June i, 1923